Increase/decrease logging level on notary-signer

[HuKeping]

For people who would like to try some contribute to Notary :)

For now we have already supported increasing/decreasing logging level on notary server side #748 , and it would be nice to have that functionality also on notary signer side.

Tips:
What you need to do would like to be:

• Embed a setupSignalTrap() into the notary-signer booting process.
• Add a handler to deal with those captured signals.
• Add some test for your handler
• Add some docs about it

The infra you might want to use for this feature are:

• NotarySupportedSignals in const.go
• AdjustLogLevel() to increase/decrease logging in utils/configuration.go

[cyli]

Fixed by #970